1. Field of the Invention
This invention relates to a voltage control oscillator for a semiconductor device, and more particularly to a voltage control oscillator which can controll the oscillation period of an oscillator by way of an input voltage.
2. Description of a Related Prior Art
Generally, a voltage control oscillator (VCO) has a plurality of inverters 10, 12, 14, 16, 18, which are connected to each other in the form of a chain, and a plurality of transistors 20, 22, 24, 26, 28 each of which is connected to a supply voltage input terminal of each inverter as shown in FIG. 1. A supply voltage applied to the inverters 10, 12, 14, 16, and 18 controls an output period of a ring oscillator. It is desirable that the number of inverters be an odd number. The voltage control oscillator obtains a desired period by controlling a gate voltage of the transistors 20, 22, 24, 26, 28.
However, it is necessary to closely control the gate voltage to obtain a desired period. Also, a separate buffer is necessary to maintain a constant swing width since noise margin decreases with variation of swing width.